In a modern X-ray CT apparatus, X-ray detectors and arithmetic processors have been operated at higher speeds and enhanced in performance. With this trend, real-time display of X-ray CT images has been enabled by high-speed image reconstruction performed concurrently with collection of X-ray data.
Furthermore, dynamic computer tomography utilizing such high-speed tomographic technique has been developed and already put into practical use in clinical sites. The dynamic computer tomography includes consists of tomographically imaging slice positions plural times and observing changes of CT values with time in the images in a real-time. Especially, in contrast enhanced dynamic CT using a contrast medium, computational processing is performed on the basis of information indicating how CT values change with time, the CT values indicating the amount of the contrast medium flowing through a blood vessel. A parameter such as the flow rate of blood is calculated and visualized. Thus, the kinetics of blood circulation through the body is observed.
Contrast media used in contrast enhanced dynamic CT are classified into a type such as xenon-based contrast media that ooze out from blood capillaries and are accumulated in tissues in the human head and another type such as iodinated contrast media that do not ooze out from blood capillaries. Recently, the latter iodinated contrast media have been used.
In contrast enhanced dynamic CT using an iodinated contrast medium, the medium is first injected from an elbow vein, and then CT scanning is started after a lapse of a given time T1. After a lapse of a given time T2, the scanning is ended. This time T1 corresponds to the time required for the contrast medium injected in the elbow vein to reach the slice to be tomographically imaged. The time T2 corresponds to the time taken for the contrast medium to flow out (disappear) after entering the slice planes.
These times depend on the velocity of blood stream and therefore are different among patients. Consequently, taking account of the range of variations among patients, T1 is empirically set to the value in the shortest case. T2 is empirically set to the value in the longest case.
According to this method, a large margin is contained in the period from the beginning to the end of the CT scanning. There is the possibility that the X-ray dose to the patient is high.
A first method for solving this problem is to perform X-ray scanning at low-dose at slice positions to be diagnosed. If the contrast medium appears in the blood vessels in the CT images at the given slice positions obtained by the low-dose X-ray scanning, the scanning is switched to X-ray scanning using a high-dose. However, the timing of arrival cannot be easily known because signs or symptoms of arrival of the contrast medium cannot be known from the images.
On the other hand, in a second method described in Japanese Patent Publication (Kokai) No. 11-342125, a pilot image at the slice position is taken tomographically. A region of interest (hereinafter mentioned as the “ROI”) is set in the area showing blood vessels of the pilot image data. Low-dose X-ray scanning is performed at this slice position. The CT value at the ROI of the image data is calculated. When this CT value exceeds a preset threshold value, the scanning is automatically switched to high-dose X-ray scanning.
In a third method described in Japanese Patent Publication (Kokai) No. 6-114049, the timing at which the CT value at a given ROI of image data obtained by low-dose X-ray scanning exceeds a first threshold value is automatically detected in the same way as the foregoing method. On the basis of the detected signal, high-dose X-ray scanning is started. Furthermore, the timing at which the CT value at the ROI on the images obtained by the high-dose X-ray scanning decreases below a second threshold value is similarly automatically detected. Thus, CT scanning is ended.
In the second and third methods described above, the apparatus compares the CT values of blood vessel regions with a preset threshold value. Thus, the timing at which high-dose X-ray scanning is started or ended is automatically set. In practice, however, it has been difficult to precisely judge the timing at which high-dose X-ray scanning is started or ended from the comparisons with the uniquely defined threshold values, because peak CT values and profiles of the time-density curve (hereinafter mentioned as the “TDC”) of CT values are different among patients. Accordingly, it has been difficult to precisely judge the timing of irradiation in high-dose X-ray scanning (especially, the timing of the end of irradiation). There is the possibility that the patient receives a high-dose of X-rays in high-dose X-ray scanning.